ZONAL ORGANIZATION OF CORTICO-NUCLEAR AND NUCLEO-CORTICAL PROJECTIONSOF THE PARAMEDIAN LOBULE OF THE CAT CEREBELLUM - 1 - THE C-1 ZONE

The cortico-nuclear (C-N) and nucleo-cortical (N-C) projections of the C-1 cortical zone in pars anterior (pa) and pars copularis (pc) of th e paramedian lobule (PML) in the posterior lobe of the cat cerebellum were investigated with a combined electrophysiological and neuroanatom ical technique. In each experiment the mediolateral boundaries of the zone were located on the cortical surface by recording field potential s mediated via climbing fibres and evoked in the zone by activity elic ited in spino-olivocerebellar paths through percutaneous stimulation o f-fore-and hindlimbs; a small (15-30 nl) injection of WGA-HRP was then made into the zone. The distributions in the deep cerebellar nuclei w ere determined (with light microscopy) both for terminal labelling due to anterograde axonal transport by Purkinje cells and for cell bodies labelled due to retrograde transport in N-C axons. The extent to whic h injection sites were confined to the C-1 zone was assessed both by c omparing injection site and zone widths and by determining the distrib utions of retrogradely labelled neurones within the contralateral infe rior olive. The C-N projection from the part of the zone in PML pa (a forelimb part) terminates almost exclusively (perhaps exclusively) in nucleus interpositus anterior (NIA), primarily in caudal and dorsal pa rts, where it overlaps heavily with the C-N projections from the lobul e V parts (also forelimb parts) of the C-1 and C-3 zones as previously defined. The C-N projection from the part of the zone in PML pc (a hi ndlimb part) also terminates virtually exclusively in NIA but primaril y in almost all parts of the medial third of the nucleus. There is, ne vertheless, sufficient overlap between the PML pa and PML pc projectio ns that approximately one third of the termination territory of each p rojection overlaps that of the other. The PML pc part of the zone is a lmost entirely lacking in a N-C projection, as previously found for th e lobule V part of the C-1 zone (and C-3 zone). However, the PML pa pa rt of the zone receives N-C projections that arise, in descending orde r of size, from nucleus interpositus posterior (NIP), from NIA, from t he NIA/nucleus lateralis (NL) fusion area and (perhaps) NL. The projec tion from NIP is similar in size to that provided by the nucleus to th e C-2 zone in lobule V of the anterior lobe. The findings are discusse d, with particular emphasis on their implications for the hypothesis t hat the cerebellum is divisible into a number of olivo-cortico-nuclear complexes or compartments.